In cryptography, as in life the order in which you do certain things makes a dif

Question

In cryptography, as in life the order in which you do certain things makes a difference and sometimes it doesnot. For each of the following statements determine whether or not assertion is true and if it is prove that it is and if not prove that it is not.

a) DES operating on a general plaintextr input first with key K1 and then with key K2 produces the same output as if K2 were first used and then K1.

b) Starting with a general English plaintext P and two monoalphabetic substitution schema, M1 and M2, the outcome of M2 applied to (M1 applied to P) is the same as the outcome of M1 applied to (M2 applied to P)

c) Using two keys K1 and K2 and the Vigenere polyalphabetic method, the result of encrypting a general plaintext fuirst with K1 and then encrypting the result with K2 is the same as first encrypting the plaintext with K2 and the result with K1.

Explanation / Answer

a.False.

The heart of the DES is the DES-function which generates different keys. In the DES function, the key is first reduced to 32 bits and is EX-ored with the right half of the text. Then, it is passed through s-box followed by a straight p-box. These operations produces different results for different key as s-box are non-linear and there is no relation between key and s-box output.

b. False.

It can be observed with the affine cipher. Suppose the result of applying addition first and then multiplication is A1. If we apply multiplication first and then addition, the result will be different and not equal to A1.

c. True.

Since vigenere cipher is an extended additive or ceaser cipher, there is no effect if we reverse the order of keys. The result will be same in both the cases.

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